Bill Houghton, Sebastopol, CA
>The main safety issue is if the PLUMBING becomes energized - say, a hot wire touches it due to chafing, or there's an internal fault in some water-connected appliance like a dishwasher - you want it to conduct the power to ground (and, one hopes, trip the relevant circuit breaker), rather than you conducting it to ground.

Sorry - it's been a long, long day at the end of a long, long week.